1. Start enrichment
The basic injection duration cannot be calculated from the amount of the intake air because the engine speed is low and the changes in the amount of the intake air are large at starting. For this reason, the fuel injection duration at starting is determined from the coolant temperature. The coolant temperature is detected by the water temperature sensor. The lower the water temperature is the fuel vaporization becomes worse. Therefore, the air-fuel mixture is made richer by lengthening the injection duration. The engine ECU determines that the engine is being started when the engine speed is 40rpm or less. In addition, when the engine speed suddenly falls below 40rpm due to a sudden increase of the load on the engine, a hysteresis is used to prevent the engine ECU from determining that an engine that has already been started is being started again unless the engine speed falls below 20rpm.
When there is a malfunction with the water temperature sensor, it can be considered as the worse startability.
To improve startability while the engine was cold, the old type of EFI had a cold start injector and cold start time switch in addition to the regular injector to increase the fuel volume at starting.
2. Warm-up enrichment
The amount of the fuel injection is increased because the fuel vaporization is poor during the cold engine. When the coolant temperature is low, the fuel injection duration is increased to make the air-fuel mixture richer in order to attain the drivability during the cold engine. The maximum correction is twice as long as normal temperature.
When there is a malfunction with the water temperature sensor, it can be considered as poor drivability.
3. Air-fuel ratio feedback correction (For most models)
When there are no major fluctuations in the engine load or engine speed, such as when idling or driving at constant speed after warming up, fuel (air-fuel mixture close to the theoretical air-fuel ratio) is supplied based on the amount of the intake air. The following corrections are activated when driving at a constant speed after warming up.
(1) Feedback control using the oxygen sensor (Air-fuel ratio feedback control)The engine ECU determines the basic injection duration to achieve the theoretical air-fuel ratio. However, a slight deviation from the theoretical air-fuel ratio occurs in accordance with the actual engine conditions, changes over time, and other conditions. Therefore, an oxygen sensor detects the oxygen concentration in the exhaust gas to determine if the current fuel injection duration becomes the theoretical airfuel ratio against the amount of the intake air. If the engine ECU determines from signals of the oxygen sensor that the air-fuel ratio is richer than the theoretical air-fuel ratio, it shortens the injection duration to make the air-fuel mixture leaner. Conversely, if it determines that the air-fuel ratio is lean, it will lengthen the injection duration to make the air-fuel mixture richer. The feedback control operates to maintain the average air-fuel ratio at the theoretical air-fuel ratio by repeatedly performing minor corrections. (This is called a closed-loopoperation.)
In order to prevent overheating of the catalyst and assure
good engine operation, air -furl ratio feedback does not
occur under the following conditions (open-loop operation):
During engine starting
During after-start enrichment
During power enrichment
When the coolant temperature is below a determined level
When fuel cut-off occurs
When the lean signal continues longer than a determined
time The center point (a) changes during the feedback control such as time passes. In this case, the center point is forced to be returned to the center. If it is not, it will cause the out of the correction range of the feedback control. This is called air-fuel ratio learned control or long fuel trim.
(2) Feedback control using the air-fuel ratio sensor (A/F sensor):
The output voltage of the oxygen sensor changes rapidly around the theoretical air-fuel ratio as shown in the illustration (upper). The A/F sensor data which the engine ECU attains is displayed in the hand-held tester. (When the air-fuel ration is lean, the voltage is high. Conversely, the voltage is low when rich.) As a result, the detection precision of the air-fuel ratio has been improved. If the current air-fuel ratio changes from the theoretical air-fuel ratio as shown in the illustration (below), the engine ECU continuously corrects the air-fuel ratio using the oxygen sensor signal. For the A/F sensor, however, the engine ECU corrects instantly by determining the amount of change from the theoretical air-fuel ratio.
(3) CO emission control correction for vehicles without an oxygen sensor or A/F sensorFor vehicles without an oxygen sensor or A/F sensor, a variable resistor can be used to adjust the CO concentration (%) during idling. Turning the resistor to the R side makes the concentration richer, and turning it to the L side makes it leaner. For vehicles equipped with an oxygen sensor or A/F sensor, however, CO adjustment is not required during idling because these vehicles are automatically adjusted to the proper air-fuel ratio using the sensor signal.